Magneto-optical recording medium and method of making same

ABSTRACT

An improved magneto-optical recording medium and a method of making the same, the medium including a substrate base and a magneto-optical recording layer formed on the base and composed of a plurality of layers each of which contains a rare earth metal and a transition metal. The relative concentration ratio between the rare earth metal and the transition metal in the layers varies cyclically in the direction of the thickness of the magneto-optical recording layer. The recording medium is conveniently manufactured by simultaneously depositing the transition metal and the rare earth metal by co-sputtering the two metals on a base and providing relative movement between the base and the metal sources during co-sputtering to produce the multilayer recording structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of magneto-optical recording mediacontaining a recording layer composed of a plurality of individuallayers each of which contains a rare earth metal and a transition metalwhich vary in relative proportion between the individual layers.

2. Description of the Prior Art

There are magneto-optical disks suggested in the prior art which can berecorded on and/or read out by a laser beam such as a semiconductorlaser beam. Such magneto-optical disks are commonly made of an amorphousalloy of a rare earth metal and a transition metal. A "transition metal"is defined as a metal in which the available electron energy levels areoccupied in such a way that the d-band contains less than its maximumnumber of 10 electrons per atom and includes metals such as iron,cobalt, nickel, and tungsten.

It is common to use a sputtering process to obtain such amagneto-optical disk for use as a magneto-optical recording medium. Inthe past, a single sputtering source has been used to accomplish thisresult. The sputtering source or target is constructed such that pelletsor other thin pieces of rare earth metal are located on a transitionmetal target. These metals are then simultaneously deposited on asubstrate or base through a sputtering process.

FIG. 1 of the drawings illustrates a magnetization graph which plotsintensity of magnetization against the intensity of the magnetic fieldof the magneto-optical recording medium thus made. As shown in FIG. 1,the prior art magneto-optical recording medium has a poor squarenessratio in its hysteresis characteristic. Accordingly, when an externalmagnetic field is applied to the record medium, the recorded level ofthe medium is deteriorated and hence the S/N (signal-to-noise) ratio islowered. In addition, the conventional sputtering method has a defect inthat it is quite difficult to obtain the magneto-optical recordingmedium as a uniform sputtered layer which has uniform magneticcharacteristics over the entire area of the record medium.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved magneto-opticalrecording medium which can eliminate the above defects inherent in theprior art and its manufacturing method. In the present invention, thereis provided a magneto-optical recording medium having uniform andimproved magnetic characteristics at each portion thereof as well as animproved manufacturing method. The present invention provides amagneto-optical recording medium having a high coercive force, excellentsquareness ratio, a high S/N ratio and an improved manufacturing method.

In accordance with the present invention, we provide a magneto-opticalrecording medium comprising a substrate base, and a magneto-opticalrecording layer formed on the base, the recording layer containing arare earth metal and a transition metal whose relative concentrationratio is cyclically varied in the direction of thickness of themagneto-optical recording medium. To put it another way, the relativeconcentration of each metal varies sequentially above and below anaverage value along the thickness dimension of the recording layer.

The present invention is also involved with a method of manufacturing animproved magneto-optical recording medium which method includes thesteps of providing both a transition metal target or source and a rareearth metal target or source opposite to the base, the sources being inspaced relation to each other, depositing the transition metal and therare earth metal simultaneously from the respective sources on the baseby a co-sputtering process so that the sputtered positions do notcoincide and providing relative movement between the base and the metalsources to form on the base a magneto-optical recording layer whichcontains the rare earth metal and transition metal with the compositionratio between the two varying cyclically in the direction of thethickness of the magneto-optical recording layer.

Other objects, features and advantages of the present invention willbecome apparent from the following description taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a magnetization curve of a conventional magneto-opticalrecording medium of the prior art;

FIG. 2 is a schematic, enlarged cross-sectional view showing anembodiment of the magneto-optical recording medium according to thepresent invention;

FIGS. 3A and 3B are respectively graphs showing the distribution ofcomponents of the magneto-optical recording medium shown in FIG. 2;

FIG. 4 is a schematic diagram of an example of a sputtering apparatuswhich can be used to carry out the manufacturing method of the presentinvention;

FIG. 5 is a plan view on an enlarged scale of a portion of the apparatusshown in FIG. 4;

FIG. 6 is a magnetization curve of the magneto-optical recording mediumaccording to the present invention;

FIGS. 7 and 8 are magnetization curves useful for explaining the presentinvention; and

FIG. 9 is a graph showing the relationship between saturatedmagnetization and temperature for the magneto-optical recording mediumof the present invention, and the medium of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be further described in connection with thedrawings accompanying this application.

In accordance with the present invention, as shown in FIG. 2, there isprovided a substrate 1 in the form of a disk or the like and consistingof glass, an acrylic resin, a polycarbonate resin, or the like. On thesubstrate 1 there is deposited a magneto-optical recording layer 2containing elemental metal substances or alloys of rare earth metalssuch as terbium (Tb) or gadolinium (Gd) or the like. The recording layeralso contains elemental metals or alloys of transition metals, such asFe, Co, Ni, or the like, the former two being preferred, in analternating multilayer structure which in combination with the substrate1 produces a magneto-optical recording medium 3. In accordance with thepresent invention, the relative compositional ratio between the rareearth metal and the transition metal in the recording layer 2 iscyclically changed along the direction of the thickness of the recordinglayer 2. In other words, the concentration of each of the two metalsvaries above and below an average value along the thickness dimension ofthe layer. This is particularly illustrated in FIGS. 3A and 3B whichshow the relative amounts of rare earth metal and transition metalrespectively through the individual layers. It will be noted that therespective concentrations of rare earth metal and transition metal areperiodically varied in such a fashion that one is large while the otheris small and vice versa.

In accordance with the present invention, the rare earth metal and thetransition metal are provided from different sputtering sources fromwhich both metals are deposited on the substrate base by a co-sputteringprocess simultaneously, to produce the magneto-optical recording medium3. In order to provide separate sputtered positions for both thesputtering sources, the relative position between the sputtering sourcesand the base and also the mask form disposed between the sputteringsources and the base are selected appropriately. Then, by rotating thesputtering sources relative to the base 1, the rare earth metal layers 4and the transition metal layers 5 are deposited on the base in analternating multilayer structure as shown in FIG. 2 so that thecompositional ratio between the rare earth metal and the transitionmetal is cyclically changed in the direction of thickness of therecording layer 2. It is desirable for each of the rare earth metallayers 4 and each of the transition metal layers 5 to diffuse therespective metals into the previously deposited and adjacent transitionmetal layer 5 and a rare earth metal layer 4 so that a layer composedonly of rare earth metal or only of transition metal is precluded frombeing formed.

It is also important that the growing speed of each of the metal layersproduced by the sputtering process be controlled to the range of 2 to 20Å/sec, particularly 5 to 10 Å/sec, and the relative rotational speedbetween the base 1 and the sputtering sources and other sputteringconditions are appropriately adjusted to achieve this effect. It hasbeen confirmed that if the relative rotational speed between the base 1and the sputtering sources is too rapid and the growing speed of eachmetal layer becomes less than 2 Å/sec, the transition metal and the rareearth metal are deposited only with difficulty in a multilayerstructure. It has also been confirmed that if the growing speed exceedsabout 20 Å/sec, the magnetic characteristics of the layer thus producedare lowered.

The sputtering apparatus used for manufacturing the improved recordmedium of the invention can be of the magnetron type but in accordancewith the present invention, the sputtering apparatus is made into aspecial arrangement. FIG. 4 illustrates schematically an example of asputtering apparatus for carrying out the method of the presentinvention. There is provided within an evacuated enclosure such as abell jar (not shown) a turntable 6 which is disposed for rotation aboutan axis O-O'. A base 1 made of glass, resin, or the like, is mounted onthe lower surface of the turntable 6 for deposition of the metals oralloys thereon. A pair of sputtering sources 7 and 8 are disposed inopposed relation to the base 1 with an equal angular spacing such as anangular spacing of 180° about the axis O-O'. A mask 9 is disposedbetween the sputtering sources 7,8 and the turntable 6 carrying the base1 to restrict sputtered patterns of metals from the sputtering sources 7and 8. The sputtering source 7 includes a target 10 formed, for example,of a disk-shaped plate composed of the rare earth metal Tb or an alloythereof, while the sputtering source 8 includes a a target 11 formed,for example, of a disk-shaped plate made of a transition metal such asFe or an alloy thereof. Reference numerals 12 and 13, respectively,designate magnets located beneath the targets 10 and 11.

As shown in FIG. 5, the mask 9 is provided at those portions opposite tothe targets 10 and 11 with a pair of two windows 14 and 15 of a bellshape extending in expanding relation toward both ends of a straightline shown by reference character x which passes through the centers ofthe targets 10 and 11. When the turntable 6 is stopped in rotation, therare earth metal is deposited from the target 10 on mainly one-halfportion of the base 1 through the window 14 while the transition metalis deposited from the other target 11 mainly on the other half portionthrough the window 15 by the sputtering process. Then, a D.C. sputteringprocess is employed with the targets 10 and 11 being used as negativeelectrodes while rotating the turntable 6.

In FIG. 4, reference numeral 16 indicates a shutter located between themask 9 and the turntable 6 which is used to prevent the sputtering fromthe targets 10 and 11 from arriving at the base 1 before the targets 10and 11 are heated to a suitably high temperature.

In one example of a manufacturing method according to the presentinvention, a sputtering process was employed in which the turntable 6was rotated at a speed of one rotation/3 seconds for 6 minutes, namely,for 120 rotations. In this case, a layer of 1000 Å in thickness wasdeposited on the base as a whole. The metal layer thus formed consistedof a recording layer containing both components of Tb, and Fe, with thecompositional ratio thereof changing cyclically as indicated in FIGS. 2and 3. In this case, the magnetization curve indicated an improvedsquareness ratio and coercive force Hc as shown in FIG. 6.

The reason that the squareness ratio and the coercive force Hc areimproved can be described as follows. In general, the coercive force Hcdepends on magnetic anisotropy and magnetostriction. In themagneto-optical recording medium of the present invention, the magneticanisotropy is provided by the mutual action of both components in thealloy composed of the rare earth metal and the transition metal. Themagnetostriction is generated inside the layer by the difference incomposition ratios of both components in the direction of the thicknessof the recording layer. The magnetic anisotropy and the magnetostrictionthus generated may be considered as contributing to the improvements ofthe coercive force Hc and the squareness ratio.

It is desirable that the sputtering process employed by a D.C.sputtering method. FIGS. 7 and 8 are curves showing the respectivemagnetization curves of the recording media formed by a radio frequencysputtering method and a D.C. sputtering method, respectively. Bycomparing FIGS. 7 and 8, it will be clear that the magnetization curvein the case of the D.C. sputtering method provides a better squarenessratio characteristic (FIG. 8) than in the case of the radio frequencysputtering method (FIG. 7).

While in the above embodiments, the rare earth metal and the transitionmetal are used in elemental form, one or both may be in the form of acombination of two or more elements. For example, a TbGd alloy may beused as a rare earth metal source and an FeCo alloy as the transitionmetal source. When the rare earth metal and/or the transition metal arecomposed of a metal containing two elements or more, the followingmethod can be used. The target 10 and/or 11 is formed of theabove-mentioned alloy. The portion facing the window 14 and/or 15 of themask 9 is arranged such that the ratio between the areas of therespective metal portions becomes equal to the composition ratio betweenthe finally deposited metals. The other metal pellet is superposed on ametal plate as the target 10 and/or 11 of the sputtering sources.

As described above, the rare earth metal and the transition metal aredeposited from different sputtering sources. Each of the metals may beprovided from a plurality of sources.

As set forth above, the present invention makes it possible to obtain amagneto-optical recording medium having a high coercive force and anexcellent squareness ratio, and thereby provide a high S/N ratio.

Furthermore, the anisotropy constant Ku of the layer magnetized normalto the surface of the magneto-optical recording layer is on the order of10⁶ erg/cc which is higher than that of the prior art recording layer byone order of magnitude. In other words, according to the presentinvention, the coercive force Hc is raised so that the recording layerhas an excellent life and thermal stability, and a higher densitymagnetic recording becomes possible. More specifically, the minimumdiameter d of a recorded bit is determined by: ##EQU1## where Ew is themagnetic wall energy and Ms is the saturated magnetization. Thus, thelarger coercive force value Hc makes the diameter d of the recorded bitsmaller so that the recording density can be improved.

In addition, the saturated magnetization-temperature characteristic ofthe prior art magneto-optical recording layer exhibits ferromagnetism asshown by the broken line in FIG. 9 so that the prior art magneto-opticalrecording medium when used at room temperature is in the vicinity of itscompensation temperature Tcomp. In contrast, since the saturatedmagnetization-temperature characteristic of the magneto-opticalrecording layer according to the present invention has a magneticcharacteristic shown by the solid line in FIG. 9, and its compensationtemperature is considerably lower, the stability of the magneticcharacteristic and the intensity of magnetization can be improved.

The above description provides a single preferred embodiment of theinvention, but it will be apparent that many modifications andvariations can be effected by one skilled in the art without departingfrom the spirit or scope of the novel concepts of the invention, so thatthe scope of the invention should be determined by the appended claimsonly.

We claim as our invention:
 1. A method for manufacturing amagneto-optical recording medium comprising the steps of:providing atransition metal source and a rare earth metal source in spacedrelation, simultaneously depositing said transition metal and said rareearth metal by co-sputtering the two metals on a base, and providingrelative movement between said base and said source during suchco-sputtering to produce a multilayer recording layer in which therelative concentration between said rare earth metal and saidtransmission metal varies cyclically in the thickness dimension of therecording layer.
 2. A method according to claim 1 in which:said layersare deposited at a deposition speed of from 2 to 20 Å/sec.
 3. A methodaccording to claim 1 in which said layers are deposited at a depositionspeed of from 5 to 10 Å/sec.
 4. A method according to claim 1 whereinsaid relative concentration of said metal varies sequentially above andbelow an average value along the thickness dimension of said recordinglayer.